Heretofore, it has been known to prepare a scandate cathode by impregnating a porous tungsten billet with a barium emitter such as Ba.sub.3 Al.sub.2 O.sub.6 ; then, placing Sc.sub.2 (WO.sub.4).sub.3 on top of the impregnated billet and then heating to form free barium in the impregnated billet. The free barium is formed according to the reaction EQU 2Ba.sub.3 Al.sub.2 O.sub.6 +W.fwdarw.2BaAl.sub.2 O.sub.4 +BaWO.sub.4 +3Ba
The free barium atoms that are generated migrate to the surface of the billet and react there with the Sc.sub.2 (WO.sub.4).sub.3 coating according to the reaction EQU 3Ba+Sc.sub.2 (WO.sub.4).sub.3 .fwdarw.3BaWO.sub.4 +2Sc
The difficulty in the above reaction is that Sc and BaWO.sub.4 form in the mole ratio of 2:3. If the molar ratio of BaWO.sub.4 to Sc is 3:2, then two BaWO.sub.4 is interacting with 2Sc with 1BaWO.sub.4 not interacting. Since five molecules are present (3BaWO.sub.4 and 2 Sc) and only four can interact (2 BaWO.sub.4 with 2Sc) then 1/5 or 20 percent of the surface is idle or inert.